Wind turbine control system comprising improved upsampling technique

ABSTRACT

A wind turbine control unit includes an upsampling module that receives a first control signal that includes a current control sample value and a predicted control trajectory. The upsampling module also calculates a second control signal in dependence on the current control sample value and the predicted control trajectory. The second control signal has a higher frequency than the first control signal. The upsampling module further outputs the second control signal for controlling an actuator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/303,850 filed Nov. 21, 2018, which is a U.S. National StageEntry of PCT/DK2017/050167 filed on May 22, 2017, which claims priorityto Danish Patent Application PA 2016 70350 filed on May 25, 2016. Eachof these applications are hereby incorporated by reference in theirentirety.

TECHNICAL FIELD

The present invention relates generally to a wind turbine control systemin which an upsampling technique is used to increase the signal ratebetween an actuator system and a control unit that controls saidactuator system.

BACKGROUND

A wind turbine comprises multiple systems all of which must becontrolled to function together so that the wind turbine provides atarget power output in a wide range of wind conditions. In this contextit is possible that a control unit for a given actuator system providesa digital control signal which does not match the required input rate ofthe actuator system. For example, a pitch actuator system comprising ahydraulic actuator and an actuator position control unit may require arelatively high input signal rate whereas a pitch control unit thatsends pitch position commands to the pitch actuator system provides anoutput signal at a relatively low rate. In such a case, it is necessaryto convert the relatively low rate control signal from the pitch controlunit to a higher rate signal so that it can processed correctly by thepitch actuator system. Such signal rate conversion is achievedconventionally by a suitable upsampling technique, in which the outputsignal of an upsampler includes the existing samples of the input signalas well as new samples inserted between the existing samples accordingto a predefined integer conversion factor.

Known approaches to signal upsampling include zero-order hold and zerostuffing. In a zero-order hold technique, the additional samplesinserted between the existing samples are given a value equal to theimmediately preceding existing sample, whereas in a zero-stuffingtechnique, those additional samples are given a value of zero. In bothapproaches, a low-pass post-filter serves to smooth out discontinuitiesin the signal and avoid aliasing. Although filtering in theory addressesthe aliasing issue, aliasing can still occur and, moreover, thefiltering introduces a phase delay in the control signal which isundesirable in the context of controlling a dynamically changing system.

Against this background, the present invention aims to provide animproved upsampling methodology suitable for use within a control systemin a wind turbine.

SUMMARY OF THE INVENTION

In a first aspect, embodiments of the invention provide a wind turbinecontrol unit comprising: a control module configured to control anactuator system by outputting a first control signal, wherein the firstcontrol signal includes a current control sample value and a predictedcontrol trajectory; the control unit further comprising an upsamplingmodule configured to receive the first control signal from the controlmodule, and to output a second control signal for controlling theactuator system, the second control signal having a higher frequencythat the first control signal. The upsampling module calculates thesecond control signal in dependence on the current control sample valueand the predicted control trajectory.

The invention can also be expressed as a method of operating a controlunit of a wind turbine control system to control an actuator systemthereof, the method comprising generating, using a control module, afirst control signal comprising a current control sample value and apredicted control trajectory; and generating, using an upsamplingmodule, a second control signal for controlling the actuator system, thesecond control signal having a higher frequency than the first controlsignal; wherein the upsampling module calculates the second controlsignal in dependence on the current control sample value and thepredicted control trajectory.

The invention also extends to a wind turbine control system comprising acontrol unit as defined above, and also to a computer program productdownloadable from a communications network and/or stored on a machinereadable medium, comprising program code instructions for implementingthe method as defined above.

The second control signal may comprise a first control sample value thatcorresponds to a current control sample value of the first controlsignal, and one or more further control sample values based on thepredicted control trajectory.

A benefit of the invention is that the relatively slow control signaloutput by the control module is upsampled into a faster version of thatsignal using an approach that is based on the predicted controltrajectory that is output by the control module. That is to say, thecontrol sample values that are added to existing control samples or‘control moves’ generated by the control module are based on knowledgeof the predicted control trajectory. This provides a more accuratelyreproduced control signal at a higher frequency that is suitable foronward processing which does not suffer from the problems of aliasingand delay that exist with conventional upsampling techniques. Thedynamic response of the actuator system is improved such that itexhibits lower overshoot and is more optimally damped.

The upsampling module may calculate the one or more further controlsample values using an interpolation function applied to the currentcontrol sample value and one or more sample values of the predictedcontrol trajectory and which is based on a ratio of sampling rates ofthe control module and the actuator system. The interpolation functionmay include a first order interpolation function that uses a singlesample value of the predicted control trajectory, in particular a singlesample value that immediately follows the current control sample value.Alternatively, the interpolation function may include a second orderinterpolation function that uses two sample values of the predictedcontrol trajectory, in particular two sample values that immediatelyfollow the current sample value.

In one embodiment, the control module comprises a receding horizoncontrol algorithm which calculates repeatedly a predicted controltrajectory with respect to each occurrence of a current control sample.Moreover, a model predictive control (MPC) routine may be employed.

In one embodiment, the actuator system includes at least one pitchactuator for controlling the pitch of a respective one or more windturbine blades.

Within the scope of this application it is expressly intended that thevarious aspects, embodiments, examples and alternatives set out in thepreceding paragraphs, in the claims and/or in the following descriptionand drawings, and in particular the individual features thereof, may betaken independently or in any combination. That is, all embodimentsand/or features of any embodiment can be combined in any way and/orcombination, unless such features are incompatible. The applicantreserves the right to change any originally filed claim or file any newclaim accordingly, including the right to amend any originally filedclaim to depend from and/or incorporate any feature of any other claimalthough not originally claimed in that matter.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a wind turbine in which embodiments of the invention may beincorporated;

FIG. 2 is a schematic view of a control system in accordance with anembodiment of the invention;

FIG. 3 illustrates a control trajectory as determined by a ModelPredicted Control (MPC) algorithm;

FIG. 4 is a process flow diagram in accordance with an embodiment of theinvention; and,

FIG. 5 is a series of data plots that illustrates an upsamplingmethodology in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a wind turbine 10 in which an embodiment of the inventionmay be incorporated. The wind turbine 10 comprises a tower 12 supportinga nacelle 14 to which a rotor 16 is mounted. The rotor 16 comprises aplurality of wind turbine blades 18 that extend radially from a hub 20.In this example, the rotor 16 comprises three blades 18 although otherconfigurations are possible.

FIG. 2 shows a wind turbine control system 22 in accordance with anembodiment of the invention which may be implemented in the wind turbine10 of FIG. 1 . Here, the control system 22 includes an actuator system24 that is controlled by a control unit 26. In this particularembodiment, the actuator system 24 may be a pitch system for controllingthe pitch of one or more of the wind turbine blades 18 which may includea hydraulic actuator 28 arranged to adjust blade pitch in a knownmanner. The actual position of the actuator 28 is controllable by anactuator position control unit 30 which provides a positioning commandsignal to the hydraulic actuator 28, typically at a high frequency rate,for example in the order of 100 Hz or higher.

It should be appreciated that the control unit 26 and actuator system 24may be replicated for each of the blades 18 of the wind turbine 10 sothat the position of each blade 18 may be controlled independently.

It should be noted at this point that the pitch system of the windturbine 10 is just one example of a wind turbine system that could becontrolled and that the control unit 26 could also be used to controlother wind turbine systems. For instance, the actuator system 24 may bean electric or hydraulic yaw drive for the nacelle 14 of the windturbine 10 to provide rotational position control of the nacelle 14 withrespect to the tower 12. Another example would be a converter controlsystem where the actuator system 24 may be a power converter of thegeneration system of the wind turbine 10 that converts AC powerdelivered by the generator to a variable-frequency AC power output via aDC link in a process known as ‘full power conversion’. The skilledperson would appreciate that the principle of the invention describedherein could be applied to any wind turbine system that requires highspeed real time control.

Returning to FIG. 2 , the control unit 26 comprises two functionalcomponents: a control module 32 and an upsampling module 33. Thesefunctional modules are illustrated separately here for convenience,although it should be appreciated that this does not imply that suchfunctions must be implemented in separate hardware or software modules.In overview, the control module 32 outputs a control signal that isgenerated using a dynamic model of the actuator system that predicts howthe system will respond to control inputs. Beneficially, the controlmodule 32 implements a receding horizon control methodology, which isalso known as a Model Predictive Control or ‘MPC’ algorithm, such as isdescribed in WO2016/023561. As is known, therefore, MPC algorithmsimplement an optimization model that yields a predicted trajectory offuture timeslots of the control signal, or ‘control sample values’ or‘control moves’, which allows the current sample value to be optimizedand implemented while keeping future time slots in account. As thecontrol module 32 implements Model Predictive Control, it therefore hasa predictive ability about the future state of the actuator system to becontrolled which has particular application in complex dynamic systemswhich are difficult for traditional PID controllers to controleffectively since they do not have such predictive functionality.Although such MPC-based controllers provide benefits in terms of theaccuracy with which an actuator system is able to be controlled, theytend to operate at a lower frequency largely due to their computationalcomplexity, compared to the actuator system being controlled. Expressedanother way, the actuator system requires a control signal having afrequency or rate that is higher, for example, by a factor of 10 or evenby a factor of 100, than that of the control signal output by theMPC-based control module.

The embodiments of the invention provide a solution to this problem byproviding the control unit 26 with the upsampling module or simply‘upsampler’ 33 which takes the relatively slow control signal output bythe control module 32 and outputs a faster version of the control signalthat is compatible with the actuator system 24. As will be appreciatedfrom the discussion that follows, the upsampling module 33 takesadvantage of the MPC approach implemented by the control module 32 byoutputting a second or ‘modified’ control signal that is based on thepredicted control trajectory generated by the control module 32. That isto say, the control sample values that are added between the existingcontrol sample values or control moves of the original control signal atthe lower frequency are based on knowledge of the control trajectorygenerated by the MPC algorithm implemented by the control module. Thisprovides a more accurately reproduced control signal at a higherfrequency that is suitable for onward processing which does not sufferfrom the problems of aliasing and delay that exist with conventionalupsampling techniques. Ultimately, the dynamic response of the actuatorsystem is improved such that it exhibits lower overshoot and is moreoptimally damped.

The implementation of the control unit 26 will now be described in moredetail with reference to FIG. 2 . The general function of the controlunit 26 is to control the actuator system 24 so that its output, that isto say the position of the pitch actuator in this particular example, isequal to a target value as determined by a higher level controller, forexample a pitch angle controller (not shown, but its presence isimplied). To this end, the control unit 26 receives an input signal froma summing junction 34 which provides the error ‘E’ between the currentstate of the actuator system indicated here as ‘S’, which in this casemay be the current actuator position, and a target value which isindicted here as T. The control unit 26 is operable to control theactuator system 24 to drive its state S, i.e. the pitch position of theactuator 28, to a value that is equal to the target value T, therebyminimising the error E.

In response to the signal E, the control module 32 calculates one ormore predicted control trajectories over a moving time horizon orwindow. The predicted control trajectory is a sequence of optimisedcontrol moves for a predetermined time horizon, calculated for a numberof discrete time steps. For example, the predicted control trajectory,u(t), may comprise a string of optimised control moves for a number ofdiscrete time steps, t=k, t=k+1, t=k+2, . . . , t=k+p, where t=k+p isthe final time step of the given time horizon, such that u(k) is thecurrent sample value, which may be expressed as follows:u(t)=u(k),u(k+1),u(k+2), . . . ,u(k+p).

This is illustrated in FIG. 3 , which shows a control trajectory thatmay be generated by way of example by the control module 32. The upperplot in FIG. 3 shows a predicted trajectory s(t) for the actuator stateS whilst the lower plot shows the control trajectory u(t) for thecontrol variable CU′. Historical values are shown as solid points andgrouped as 50, whilst predicted values are shows as outlined points andgrouped as 52.

In this example, the actuator state S is commanded to increase to apredetermined set-point whilst the control trajectory u(t) illustratesthe current and predicted future control moves required to make theactuator state meet the set point. Note that it is the control samplevalue at time point k, marked here as u(k), that is usually implementedby a downstream controller whilst the future predicted control movesk+1, k+2 etc are used by the control module 32 to optimise the nextcontrol sample value.

Returning to FIG. 2 , the control module 32 therefore outputs a firstcontrol signal u(t) to the upsampling module 33 which includes a currentcontrol sample value and one or more future control sample values orcontrol moves, also referred to collectively as a ‘predicted controltrajectory’, marked as u(k+t). It will be appreciated that the controlsignal may be output as a matrix of data points. The first or ‘original’control signal that is output by the control module 32 is at arelatively low rate, which may be approximately 10 Hz, by way ofexample. However, a signal with such a rate cannot be implementeddirectly by the actuator system 24 which requires a much faster signal,for example 100 Hz, but may be much higher.

The upsampling module 33 therefore functions to convert the lower ratefirst control signal from the control module 32 to a signal with ahigher rate that matches that required by the actuator system 24, suchthat the actuator system 24 is able to process the received signalcorrectly. For this, the upsampling module 33 implements aninterpolation function that is applied to the current control samplevalue u(k) and the one or more control moves of the predicted controltrajectory included in the first control signal u(t) from the controlmodule 32.

In this embodiment, the interpolation function includes a first orderinterpolation function to be applied to the current control sample u(k)and the first predicted control move of the predicted controltrajectory. However, in other embodiments of the invention theinterpolation function may comprise a higher order interpolationfunction such as a second or third order interpolation function.

The process 100 by which the control unit 26 controls the actuatorsystem 24 is described in more detail below.

Referring now to FIG. 4 , the process 100 is initiated at step 102 whenthe control module 32 of the control unit 26 receives a sample value aspart of the error signal E.

At step 104, the control module 32 calculates a control trajectory u(t)that is determined to minimise the error signal E in the establishedway. To this end, the control module 32 implements a Model PredictiveControl algorithm to determine a control trajectory comprising a currentcontrol sample value u(k) as the prediction origin, and a predictedcontrol trajectory, u(k+t), comprised of optimised control moves fordiscrete time steps for the specified time horizon, t=k+p The controlmodule 32 outputs this data to the upsampling module 33 at step 106.

It should be noted at this point that the control module 32 outputs thecontrol trajectory u(t) including the current control sample u(k) andthe predicted control trajectory u(k+t) as a single set of data to theupsampling module 33. However, it is also envisaged that the currentcontrol sample u(k) and corresponding predicted control trajectoryu(k+t) could be output as separate data sets. The skilled person willappreciate that the length of the predicted control trajectory willdepend on the system to be controlled, that is to say the oscillatorytime period, and the sampling rate of the control module.

At step 108, the upsampling module 33 calculates a modified or ‘second’control signal to output to the actuator system 24 which has a higherfrequency than the first control signal. Firstly, the upsampling module33 receives the current control sample, u(k), and the predicted controltrajectory, u(k+t), from the control module 32. Then the upsamplingmodule 33 uses these sample values, u(k), u(k+1), along with the knownsample rates of the actuator system 24 and the control module 32 tocalculate the modified control signal. For the purposes of thisdiscussion, the frequency of the actuator system 24 is termed f1, andthe output frequency of the control module 32 is termed f2. As has beenmentioned previously, f1>f2 for example by a factor of 10.

In general terms, rather than carry out a conventional upsamplingtechnique in which additional sample values are added at either zerovalue (zero stuffing) or at a value of the previous control sample (zeroorder hold), combined with suitable post-filtering, the upsamplingmodule 33 provides a modified signal which comprises additional samplevalues that are based on the current control sample and one or more ofthe control moves of the predicted control trajectory u(k+t). By addingsamples in the period between successive control samples sent by thecontrol module 32, the output of the upsampling module 33 has a higherfrequency. For example, if nine samples are added (10-1 samples toaccount for the existing control sample), the frequency is increased bya factor of 10 compared to the frequency of the first control signal.

To generate the modified control signal, the upsampling module 33applies a first order interpolation function to the current controlsample, u(k), and the first predicted control move, u(k+1) to deriveeach of the additional control samples.

Each addition or ‘intermediate’ control sample can therefore becalculated using the following relationship,

${{next}{intermediate}{control}{sample}} = {{{current}{control}{sample}} + {\left( \frac{{u\left( {k + 1} \right)} - {u(k)}}{\frac{f_{1}}{f_{2}}} \right).}}$

Starting from t=k, this calculation is repeated at the higher subsamplerate, f₁, until the next discrete time step of the controller, t=k+1, isreached. This results in the first predicted control move from thecontroller, u(k+1), being broken up into a number of smaller steps whichcan be executed at the higher sample rate of the actuator system, f₁.

Once the modified or ‘second’ control signal has been determined for thetime period between t=k to t=k+1, the upsampling module 33 sends themodified control signal to the actuator system at step 110, as indicatedas CM′ on FIG. 2 . At step 112, the actuator system 24 implements thecontrol moves of the modified control signal M at the actuator systemrate f₁. The process thereafter repeats for each sample data point thatis received at the control module 32 at rate f1. Thus, at time t=k+1,the control module 32 receives a new current control sample of theactuator system and steps 104 to 112 are repeated to provide theactuator system 24 with a modified control signal for the time periodt=k+1 to t=k+2.

The above process is illustrated in FIG. 5 which shows a controltrajectory u(t) at three sequential time steps k, k+1 and k+2.Considering firstly the first time step k, which is the uppermost plot,it will be seen that the second control signal (upsampled output) fromthe upsampling module 33 extends, or is interpolated, between thecurrent control sample value u(k) and the first control move u(k+1) ofthe predicted control trajectory. The number of sample values formingpart of the interpolation may be determined based on the scaling factorrequired between the control module 32 and the actuator system 24, thatis to say, the ratio of the frequency of the actuator system to thefrequency of the control module 32. For example, if the frequency of theactuator system is 100 Hz and the frequency of the control module 32 is10 Hz, then the upsampling module 33 will add nine (f1/f2−1) additionalsample values between the current control sample and the next samplefrom the control module 32.

The second and third plots in FIG. 5 show the next two successive timesteps where the same process is applied.

It will be appreciated that various modifications may be made to thespecific embodiments discussed above without departing from theinventive concept as defined by the claims.

For example, in the embodiment discussed above the additional controlsample values in the second control signal M are based on a first orderinterpolation applied on the current control sample value u(k) and thenext control move u(k+1) in the predicted control trajectory generatedby the control module 32. That is to say, only the first of thepredicted control moves are used to influence the additional controlsample values. However, in a variant of the above process, theupsampling module 33 may use a second order interpolation function tocalculate a modified control signal. In such a case, the control module32 calculates a predicted control trajectory in the same way as in thefirst embodiment, although the upsampling module 33 takes into accounttwo predicted control moves u(k+1), u(k+2) in addition to the currentcontrol sample u(k) to generate the additional control sample values.The upsampling module 33 then uses second order interpolation of theseinputs and knowledge of the difference in frequencies of the actuatorsystem and the control module to calculate the modified control signal Mfor output to the actuator system. The use of a second orderinterpolation function ensures continuity for both the actuator controlmoves and its derivative.

The invention claimed is:
 1. A wind turbine control unit comprising anupsampling module configured to: receive a first control signalcomprising a current control sample value and a predicted controltrajectory; calculate a second control signal in dependence on thecurrent control sample value and the predicted control trajectory,wherein the second control signal has a higher frequency than the firstcontrol signal; and output the second control signal for controlling anactuator.
 2. The control unit of claim 1, wherein the second controlsignal comprises a first control sample value that corresponds to acurrent control sample value of the first control signal, and one ormore further control sample values based on the predicted controltrajectory.
 3. The control unit of claim 2, wherein the upsamplingmodule calculates the one or more further control sample values using aninterpolation function applied to the current control sample value andone or more sample values of the predicted control trajectory and whichis based on a ratio of sampling rates of a control module and anactuator system comprising the actuator.
 4. The control unit of claim 3,wherein the interpolation function includes a first order interpolationfunction that uses a single sample value of the predicted controltrajectory.
 5. The control unit of claim 4, wherein the single samplevalue of the predicted control trajectory immediately follows thecurrent control sample value.
 6. The control unit of claim 3, whereinthe interpolation function includes a second order interpolationfunction that uses two sample values of the predicted controltrajectory.
 7. The control unit of claim 6, wherein the second orderinterpolation function uses the two sample values that immediatelyfollow the current control sample value.
 8. The control unit of claim 1,further comprising a control module, wherein the control modulecomprises a receding horizon control algorithm which calculatesrepeatedly a predicted control trajectory with respect to eachoccurrence of a current control sample.
 9. The control unit of claim 1,further comprising a control module, wherein the control modulecalculates the predicted control trajectory using an optimization model.10. The control unit of claim 1, further comprising a control module,wherein the control module calculates the predicted control trajectoryby implementing a model predictive control (MPC) routine.
 11. Thecontrol unit of claim 1, wherein the actuator includes at least onepitch actuator for controlling a pitch of a respective one or more windturbine blades.
 12. The control unit of claim 1, wherein the controlunit is disposed in a control system of a wind turbine.
 13. A methodcomprising: generating, using a control module of a wind turbine, afirst control signal comprising a current control sample value and apredicted control trajectory; calculating, using an upsampling module ofthe control module, a second control signal in dependence on the currentcontrol sample value and the predicted control trajectory, wherein thesecond control signal has a higher frequency than the first controlsignal; and outputting the second control signal for controlling anactuator.
 14. The method of claim 13, wherein the second control signalcomprises a first control sample value that corresponds to a currentcontrol sample value of the first control signal, and one or morefurther control sample values based on the predicted control trajectory.15. The method of claim 14, wherein the one or more further controlsample values are calculated using an interpolation function applied tothe current control sample value and one or more sample values of thepredicted control trajectory and which is based on a ratio of samplingrates of the control module and the actuator.
 16. The method of claim13, further comprising calculating repeatedly a predicted controltrajectory with respect to each occurrence of a current control sample.17. The method of claim 13, further comprising calculating the predictedcontrol trajectory using an optimization model.
 18. The method of claim13, further comprising calculating the predicted control trajectory byimplementing a model predictive control (MPC) routine.
 19. The method ofclaim 13, wherein the actuator includes at least one pitch actuator forcontrolling a pitch of a respective one or more wind turbine blades. 20.A computer program product at least one of downloadable from acommunications network and stored on a machine readable medium,comprising program code instructions for performing an operation whenexecuted on a computer process, wherein the operation comprises:generating, using a control module of a wind turbine, a first controlsignal comprising a current control sample value and a predicted controltrajectory; calculating, using an upsampling module of the controlmodule, a second control signal in dependence on the current controlsample value and the predicted control trajectory, wherein the secondcontrol signal has a higher frequency than the first control signal; andoutputting the second control signal for controlling an actuator.